In the U.S., rare earth elements (REEs) are used in a number of applications, including advanced technology products. A lack of sufficient domestic supply, however, has forced the United States to rely on foreign sources. Developing domestic sources of REEs is important to promote national security and independence, which has been well documented.
Coal is widely used in industries such as power plants, gasification, and iron & steelmaking. It contains small quantities of REEs at parts per million (ppm) levels. After coal has been used in an industrial process, REE levels are typically increased by approximately one order of magnitude as they partition to coal byproducts in the ash (or slag if the system is hot enough to melt the ash), typically ranging in total concentration between 200-400 ppm, although in some geographic locations for a coal, REE levels can reach 8,000 or higher (See Hower, J., et al., International Journal of Coal Geology, 39 (1999) 141-153 and Hower, J., et al., Coal Combustion and Gasification Products, 5 (2013) 39-47 each of which is incorporated herein by reference). Challenges to extract REEs from a coal byproduct exist partly due to the fact that coal byproduct REEs are present in various mineral forms with different properties. REEs may exist chemically as oxides, be dissolved in mineral phases, or be present in the vitrified slag phase. The low concentrations and various forms of REEs make traditional leaching extraction processes more complex and less economical as more types and quantities of acids and beneficiation stages are required to recover REEs. The present invention addresses REE recovery challenges by melting the coal ash byproduct so all the REEs are in a single form (liquid), then concentrating them into a REE enriched solid phase to facilitate extraction of REEs.
Naturally occurring REE phosphate (monazite for example), one of the predominant mineral sources of REEs, is known to exist below earth's surface in limited areas. Geologists report the formation of natural monazite requires extremely high pressures up to 8,000 atm (about 117,600 psi) and elevated temperatures depending on the chemistry of the melt and the cooling/heating conditions that REEs are subject to.
It is not known how to manufacture REE rich monazite synthetically from coal byproducts in a molten state (extremely high temperatures). This is specifically true using a low pressure procedure (under atmospheric pressure of 1 atm (14.7 psi)). In order to develop an effective and economical REE concentration method without use of high pressures, a unique process was developed.
One or more embodiments include achieving high concentration of REEs based on simple temperature manipulations in atmospheric air. One or more embodiments is practiced at atmospheric pressure, different from naturally occurring REE phosphate formation (a main source for REEs) requiring extremely high pressures.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.